Various rubber products, including tires and industrial products, are composed of elastomers, or blends of elastomers, which are conventionally reinforced with reinforcing particulate fillers to enhance their physical properties.
While carbon black is often used for such purpose, silica is also sometimes solely used or, alternatively, is used in conjunction with carbon black to reinforce the elastomers and/or to otherwise enhance their physical properties. The use of carbon black and/or silica to reinforce elastomers is well known to those having skill in such art.
Such silica is normally composed primarily of silicon dioxide which contains silanol groups on its surface as well as water of adsorption unless it is used in a dried or dehydrated form. While the silica can be, for example, of the pyrogenic or precipitated type, the precipitated silica is used in most applications.
As hereinbefore pointed out, for rubber reinforcement purposes, the silica, particularly precipitated silica, is generally considered to be in the form of aggregates of silica particles.
Thus, the term "silica", as may be referred to herein, is generally intended to mean aggregates of silica particles unless the primary silica particles are referenced.
In one aspect, in order to enhance the elastomer reinforcing effect of silica, a coupling agent, or silica coupler, is often used in conjunction with the silica. Such silica coupler is often composed of a moiety which is reactive towards the silanol groups present on the silicon dioxide surface and/or towards Si--O--Si groups, or any other active sites, such as, for example, an organosilane, and of a moiety which is reactive with the carbon-to-carbon double bonds of the elastomer such as, for example, a polysulfide or a mercaptan. The silica coupler, with one moiety reacted with the silica and one moiety reacted with the elastomer, then couples, or connects, the silica to the rubber in a manner which enables the silica to more effectively reinforce the rubber. Such use of silica couplers is considered to be well known to those having a skill in the silica reinforcement of rubber art.
Numerous coupling agents are taught for use in combining silica and rubber, such as for example, silane coupling agents which contain a polysulfide component, or structure, such as bis-(3-triethoxysilylpropyl) tetrasulfide. Such coupling agent has a silane moiety which is reactive with silicon dioxide of the silica and a tetrasulfide moiety which is reactive with carbon-to-carbon double bonds of a sulfur curable elastomer. Dithiodipropionic acid, for example, may also be considered for use as a silica coupling agent either individually or, for example, in combination with the aforesaid silane tetrasulfide coupling agent.
Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers, or elastomers, which may be reinforced with carbon black, silica or mixtures or carbon black and silica.
Various U.S. patents relating to silicas and silica reinforced tire treads include U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537; 3,884,285; 3,938,574; 4,482,663; 4,519,430; 4,590,052; 5,066,721; 5,089,554; EPO 501227-A and British 1,424,503.
For tire treads, rubbers are conventionally evaluated, selected and blended for a purpose of achieving desired tire tread properties and particularly a balance of tire tread characteristic properties, mainly, rolling resistance, traction and wear.
The elastomer reinforcing ability of aggregates of silicon dioxide is often considered to be due to the size and shape of the aggregates, their surface activity and porosity as well as various chemical and physical interactions of the silica aggregates with silica couplers and/or elastomers.
The elastomer reinforcing ability of silica aggregates tend to be different among the various types of silica aggregates which can result in substantial variations in silica reinforced elastomer processing, prior to sulfur vulcanization of the elastomer, as well as properties of vulcanized silica reinforced elastomers. Such phenomena are well known to those having skill in the silica reinforced rubber art.
It is considered herein that an important disadvantage of many silica aggregates used for tire tread elastomer reinforcement is, for example, evidenced by a difficulty providing a silica reinforced elastomer for a tire tread which will demonstrate a suitable balance between tread wear, rolling resistance and traction properties for the tire.
It is further considered herein that silica aggregates used in elastomer reinforcement are sometimes observed to be difficult to disperse sufficiently homogeneously or sufficiently efficiently within the elastomer to provide a silica reinforced elastomer for a tire tread having a suitable balance between treadwear, rolling resistance and traction for a tire.
While the mechanism or theory relating to the relationship between dispersibility of various silica aggregates in various elastomers and resultant elastomer properties may not be fully understood, it is considered herein that characteristics of various silica aggregates themselves are involved or relate to such relationship.
Silicas are typically produced by one of three processes and are generally referred to by the general process in which they are produced, namely pyrogenic, gel and precipitated silicas. For elastomer reinforcement, precipitated silicas are usually preferred.
Pyrogenic or fumed silicas may be produced, for example, by a vapor process in which silicon tetrachloride vapor is reacted with oxygen and hydrogen at elevated temperatures.
Silica gels, which may be, for example, hydrogels, xerogels or aerogels, may be produced by reacting a soluble silicate such as sodium silicate with a strong sulfuric acid. The resulting gel is washed to remove residual salt, dried and then usually micronized by steam treatment to form a hydrogel. Aerogels may be similarly produced except that the water of the gel is replaced by an alcohol following which the gel is heated to remove the alcohol under super critical drying conditions (high pressure and high temperature into an autoclave). Contrary to aerogels, xerogels are not dried under super critical conditions. They are indeed inorganic hydrated oxide precipitated from an aqueous solution and dried in air or under a vacuum.
Precipitated silicas may be produced, for example, by forming a soluble silicate by reacting particles of silicon dioxide (eg: sand) with a strong base, such as sodium hydroxide, to form after dissolution an aqueous silicate solution; followed by destabilizing the silicate solution by addition of an organic or mineral acid and/or acidifying gas such as carbon dioxide to change the pH of the mixture and cause a reaction to take place from which the silica is precipitated, substantially in the form of fine particles of silicon dioxide which may be in the aforesaid aggregate form and may appear in a gel-like form. The resulting slurry, or gel, is conventionally filtered and washed with water to remove the reaction by-product, which is the alkali metal salt of the acidification agent. The filter cake is then typically dried to yield a silica of desired aggregate particle size. There are many variations of the precipitation process involving, for example, sequential and/or intermittent additions of the silicate solution and/or acid, control of the pH, optionally stopping the acid addition and ageing of the mixture prior to resuming acid addition to reach the desired pH, ageing the gel or filter cake prior to washing with water which might sometimes be called post ageing and variations of various process temperatures and times.
A process of producing a silica may be found, for example, in European Patent publication EP-A1-170579.
The physical characterizations of the precipitated silicas, namely the aggregates thereof, can vary considerably as well as their use as reinforcement for elastomer(s) and resulting elastomer properties according to the selection of silicate reaction materials and reaction conditions.
In one aspect, it is desired to provide an aggregate of particles for the reinforcement of elastomers which are somewhat similar to aggregates of silica particles except that a portion of the silicon present in the silicon dioxide particles has been replaced with other atoms and, further, that functional groups and/or polymers have been grafted to the silica surface and/or that the aggregate has been treated with a surfactant.